By the 14th century, Cardassia had been stripped of all its natural wealth, resulting in the slow desertification of most arable land on its surface. So bad was the problem that on the Night of Talons, a military dictatorship took over the planet followed by a ten hour period of assassinating key government officials. Nevertheless, the lack of resources and worsening surface conditions continued and eventually drove the Cardassians to begin expanding outward from their world in the late 16th and early 17th centuries. (Decipher RPG module: Through a Glass, Darkly)
In the 16th century, the Bajorans made first contact with the Cardassians when several Bajoran lightships, which were designed to explore the space surrounding Bajor, reaching Cardassia after being propelled there by tachyon eddies in the Denorios Belt. In the 24th century, this fact was frequently mentioned in the propaganda broadcasts of the Klingon-Cardassian Alliance as evidence of the historic bond between the Bajoran and Cardassian peoples. (TNG - Mirror Universe novel: The Worst of Both Worlds)
With the development of warp drive, the Cardissians began seeking out new resource-rich worlds that could sustain them. Many worlds were found, some with their own population just ripe for the picking. It was shortly thereafter, in the 18th century, that the Cardassian Union was formed and many of these newly discovered worlds joined when faced with the choice of either membership or extermination. With an expanding empire, resources began to arrive on Cardassia, freeing the planet from dependency on its own scarce resources. Moreover, the Cardassians began to repair some of the damage done to their planet, as well as create new and impressive architecture in their cities.
It was in 2255 that the Cardassians came into contact with the Terran Empire. With the Terrans taking Bajor and mining it for resources to press their attacks on the Union, it was not long before the homeworld itself was threatened. In 2266 the Terran Empire had pushed to within a few light years of Cardassia and stood ready to complete its conquests. Thankfully, it was at this moment that the Klingons, led by Kor, conducted a major raid on the empire, diverting its attention and military resources. The Klingon raids would prompt a delegation to be sent to Qo'noS with the final outcome being the creation of the Alliance. The two powers would go on to defeat the Terran Empire.
With the empire conquered, resources and slaves made their way to Cardassia where the ecological damage caused by centuries of exploitation was repaired to some degree. Industrial production was moved off-world and a steady stream of fresh food and non-replicatable items came in from the former worlds of the Terran Empire. As of 2370, Legate Dukat served as the Intendant of the Cardassia system and was the representative of the Cardassian people throughout the Alliance. (Decipher RPG module: Through a Glass, Darkly)